Last time before I went into some holidays, I started a render job hoping to see a completed picture coming back. Unfortunately this wasn't the case, after 8 days not even the prepass was completed.
Cause smaller sized test renders had "normal" render times, I started some sort of investigation about render time increase when the picture size increases and found the following:
1261 x 394 => 1:14,32
1574 x 492 => 2:00,15
1968 x 615 => 3:11,48
For these 3 the render time increase (more or less) linear with the picture size which is matching with my expectation...
But the next size is still rendering: 2458 x 768 => ~20hrs for the prepass.
I guess that this picture will be completed after ~60hrs which is roughly a factor of 20 by increasing the size with a factor of ~1,56.
For these tests I have kept all parameters identical (except the size) to avoid any influence by other parameters.
The RAM utilization was so far below 35%.
Does anybody has an explanation why the render time "explodes" at a certain picture size?